This invention relates to a process for forming a rivet. In particular, it relates to a process for forming a rivet in a thin metal sheet such as is used, for example for a metal container end to which a tab or ring pull is to be fixed, typically a so-called "easy open end".
It is a particular aim of this invention to provide a process which is suitable for high production rates. It is further desirable to enable rivets of small diameter to be formed in hard materials which are not very malleable such as double reduced steel. The use of double reduced steel is becoming increasingly popular in the metal packaging industry.
A process for forming a hollow rivet is described in FR-2660220 (EP-0451013). In that document, two operations are used to form the rivet. Firstly, a completely circular boss is formed by drawing the metal in a punch, the boss having annular corrugations. The rivet is then formed from this boss and having the same centre as the boss.
This process results in considerable necking of the metal in the region of the corrugations. The main force is exerted in the central part, which can lead to excessive and badly controlled thinning of the metal. This is particularly the case during the second stage, when the corrugations are ironed until they completely disappear. This consequently increases the risk of breaking off the rivet when the rivet is staked or flattened for securing a tab onto a container end. Moreover, the metal blanks are generally lacquered prior to rivet formation, so that the process frequently results in damage to this coating. Finally, the punch operation must be fairly slow to avoid splitting of the metal.
As a result of these problems, the process of FR-2660220 is in practice only suitable for use with very malleable metals, such as aluminium. This process is therefore incompatible with high production rates, unsuitable for the formation of rivets of small diameter, and difficult to use with high carbon steel sheet, such as double reduced steel.
The present invention seeks to overcome the above problems.